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This invention relates to electro-mechanical components such as micro electrical mechanical system (MEMS) devices or MEMS arrays and particularly to electrostatically-actuated electro-mechanical components wherein undesired deflection of movable elements due to nonlinear attractive effect may be controlled, preventing device failure. Components constructed according to the invention are MEMS mirror arrays or other micromachined elements.
Conventional MEMS array structures comprise Silicon on Insulator (SOI) array structures on which is fabricated an integrated electrode array. One of the problems encountered is device failure due to uncontrolled electrostatic deflection of the movable element of the device resulting in contact with the underlying substrate during operation due to overvoltage drive or inertial ringing of the device. Failure mechanisms include voltage breakdown, mechanical stiction or welding, and mechanical deformation of hinge elements. Anyone one of these failures can render a cell useless and severely hamper the usefulness of an array.
In a commonly-owned co-pending patent application Ser. No. 10/128,368 filed Apr. 23, 2002, the structure of a MEMS device is disclosed which employs a form of a stop based on the present invention. The co-pending application is not prior art to the present invention.
In U.S. Pat. No. 6,315,423 claiming a priority date of Jul. 13, 1999, one form of mechanical stop is disclosed for a MEMS device which has high angles of displacement with highly compliant hinges. The travel stops therein are for overswing and x-axis shock protection, since the hinge design therein allows for large desired rotational motions as well as undesired multi-axis translational motions. Hence the travel stops therein are not one-dimensional stops.
A solution is needed that limits the maximum deflection of the MEMS device due to inherent electrostatic parallel plate instability without compromising device performance or manufacturability.
According to the invention, a MEMS device is provided having a fixed element and a movable element wherein one of the fixed element and the movable element has at least one radially-extended stop or overdeflection limiter. A fixed overlayer plate forms an aperture. The aperture is sized to minimize vignetting and may be beveled on the margin. Overdeflection limitation occurs during deflection before the movable element can impinge on an underlying electrode. The overdeflection limiter may be conveniently placed adjacent a gimbaled hinge.
The mechanical stop may be a plate mounted to the top of a conventional MEMS structure or integrated into a three layer MEMS structure or two two-layer MEMS structures combined. The invention will be better understood by reference to the following detailed description in connection with the accompanying illustrations.